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FABRICATION AND CHARACTERIZATION OF
HYBRID POLYMER SOLAR CELL
TOONG WAY YUN
DISSERTATION SUBMITTED IN FULFILLMENT OF THE
REQUIREMENT FOR THE DEGREE OF
MASTER OF SCIENCE
DEPARTMENT OF PHYSICS
FACULTY OF SCIENCE
UNIVERSITY OF MALAYA
KUALA LUMPUR
2012
ii
UNIVERSITI MALAYA
ORIGINAL LITERARY WORK DECLARATION
Name of Candidate: Toong Way Yun (I.C/Passport No: 850907-06-5138)
Registration/Matric No: SGR090061
Name of Degree: Master of Science (Dissertation)
Title of Project Paper/Research Report/Dissertation/Thesis (“this Work”):
FABRICATION AND CHARACTERIZATION OF HYBRID POLYMER SOLAR CELL
Field of Study: Organic Electronics
I do solemnly and sincerely declare that:
(1) I am the sole author/writer of this Work;
(2) This Work is original;
(3) Any use of any work in which copyright exists was done by way of fair dealing and for
permitted purposes and any excerpt or extract from, or reference to or reproduction of
any copyright work has been disclosed expressly and sufficiently and the title of the
Work and its authorship have been acknowledged in this Work;
(4) I do not have any actual knowledge nor do I ought reasonably to know that the making
of this work constitutes an infringement of any copyright work;
(5) I hereby assign all and every rights in the copyright to this Work to the University of
Malaya (“UM”), who henceforth shall be owner of the copyright in this Work and that any
reproduction or use in any form or by any means whatsoever is prohibited without the
written consent of UM having been first had and obtained;
(6) I am fully aware that if in the course of making this Work I have infringed any copyright
whether intentionally or otherwise, I may be subject to legal action or any other action
as may be determined by UM.
Candidate’s Signature: Date:
Subscribed and solemnly declared before,
Witness’s Signature: Date:
Name: Dr. Khaulah Sulaiman
Designation: Senior Lecturer
iii
ABSTRACT
The easy fabrication method, tunable physical and chemical properties and cost-
effective fabrication process, makes organic solar cells (OSC) very attractive in
photovoltaic application. Nonetheless, the device performance is limited due to the low
charge mobility of the organic semiconductors that results in a less efficient of charge
transport to the respective electrodes. In order to address such problems, hybrid polymer
solar cells based on bulk heterojunction (BHJ) structure, which composed of a
combination of both organic and inorganic semiconductors are employed. However, the
BHJ device performances are strongly dependent on good processing conditions,
especially enhancement of photons absorption as well as the improvement of charge
transport properties. Hence, the involved parameters and properties should be well
optimized.
This dissertation describes the study of effects of blend composition and types of
acceptor materials used on the optical, structural, morphological as well as the electrical
properties of the three different hybrid BHJ systems. The hybrid materials consist of a
blend of p-type conjugated polymer of poly(3-hexylthiophene) (P3HT) and n-type
inorganic metal oxide nanoparticles, namely, zinc oxide (ZnO), titanium dioxide (TiO2)
and yttrium oxide (Y2O3). The optical, structural and morphological characterizations of
the blend thin films using UV-Visible absorption spectroscopy, X-ray diffraction (XRD)
spectroscopy, Atomic Force Microscopy (AFM) and Field-effect Scanning Electron
Microscopy (FESEM) are discussed. Furthermore, the co-relation of the thin film
property with the device performance is presented. The results show that the device
performance has been improved by optimizing the blend composition. This is due to an
enhancement in light absorption in broader wavelength regime and improved charge
transport through the formation of interpenetrating bicontinous pathway for the holes
and electrons to reach the respective electrodes. These results are supported by the
observation of the AFM and FESEM images of the increment in RMS roughness and
formation of phase separation features in the blends. Besides, the well dispersion of
inorganic nanoparticles over P3HT yields a larger interfacial area for charge carrier
generation. Among the three hybrid systems investigated, P3HT:ZnO device performs
the best with an optimal blend composition of 3% of ZnO nanoparticles in blend.
In order to further improve the device performances, ZnO sol-gel synthesis route
has been utilized to produce a better mixing blend of P3HT and ZnO. Additionally,
several approaches have been employed, namely modifying the sol content in blends,
varying the annealing temperature, and inserting an additional ZnO buffer layer between
the active layer and cathode. An optimal annealing treatment offers improved optical
absorption properties and more uniform film surface morphology with eliminated
redundant large pores and grain agglomerations. The role of the ZnO buffer layer in the
blend system can be seen as an agent in facilitating the electron collection from the
active layer to the cathode. The results indicate that the device efficiency has been
improved by about 5 times for P3HT:ZnO sol gel device with optimized sol content
(0.1ml sol), annealed at an optimized temperature of 100°C with additional ZnO buffer
layer, compared to the P3HT:ZnO nanoparticles-based device.
iv
ABSTRAK
Fabrikasi yang mudah, ciri-ciri fizikal dan kimia bolehlaras dan kos efektif
proses fabrikasi, telah menyebabkan sel suria organik (OSC) amat menarik dalam
bidang penggunaan fotovoltaik. Walau bagaimanapun, prestasi peranti menjadi terbatas
disebabkan oleh kelincahan pembawa cas yang rendah bagi semikonduktor organik
yang mengakibatkan angkutan cas ke elektrod menjadi kurang cekap. Dalam usaha
untuk menangani masalah tersebut, sel suria polimer hibrid berasaskan struktur
simpang-hetero pukal (BHJ) yang terdiri daripada kombinasi semikonduktor organik
dan bukan organic telah digunapakai. Namun demikian, prestasi peranti amat
bergantung kepada keadaan pemprosesan yang baik, terutamnya peningkatan serapan
foton serta penambah-baikan sifat angkutan. Oleh yang demikian, parameter dan sifat
yang terlibat perlu dioptimakan dengan sebaiknya.
Disertasi ini menerangkan kesan komposisi campuran dan jenis bahan penerima
yang digunakan terhadap ciri-ciri optik, struktur, morfologi serta sifat elektrik bagi tiga
sistem hybrid BHJ berbeza. Bahan hybrid terdiri daripada campuran bahan jenis-p
polimer berkonjugat (3-hexylthiophene) (P3HT) dan bahan jenis-n nanopartikel oksida
logam bukan organik, iaitu zink oksida (ZnO), titanium dioksida (TiO2) dan yttrium
oksida (Y2O3). Pencirian optik, struktur dan morfologi dibincangkan bagi filem nipis
campuran yang menggunakan spektroskopi serapan ultraungu-cahaya-nampak (UV-
Vis), spektroskopi belauan sinar-X (XRD), mikroskop daya atom (AFM) dan
mikroskopi electron daya imbasan (FESEM). Malahan, hubung-kait antara sifat filem
nipis dengan prestasi peranti juga dibentangkan. Dapatan kajian menunjukkan bahawa
prestasi peranti telah ditingkatkan dengan mengoptimumkan komposisi campuran. Ini
disebabkan oleh peningkatan dalam penyerapan cahaya di rantau gelombang yang lebih
luas dan meningkatnya angkutan cas melalui pembentukan laluan dwi-berterusan
saling-menyusup untuk pergerakan lohong dan electron sampai ke elektrod. Hasil ini
disokong oleh pemerhatian terhadap imej AFM and FESEM yang mana terdapat
kenaikan dalam nilai kekasaran RMS dan pembentukan pemisahan fasa dalam filem
campuran. Selain itu, penyerakan nanopartikel bukan organik yang seragam dalam
P3HT menghasilkan kawasan sempadan antara-fasa yang lebih luas untuk menjana
pembawa cas. Antara tiga jenis sistem hybrid yang dikaji, peranti P3HT:ZnO
memberikan prestasi terbaik dengan 3% nanopartikel ZnO ke dalam komposisi
campuran optimum.
Selanjutnya, sintesis sol-jel ZnO telah dijalankan bagi menghasilkan suatu
campuran P3HT and ZnO yang lebih baik. Tambahan pula, beberapa pendekatan telah
diambil, iaitu dengan mengubah isi kandungan sol dalam campuran, mempelbagaikan
suhu pemanasan, dan memasukkan satu lapisan penampan ZnO di antara lapisan aktif
dan katod. Rawatan pemanasan yang optimum menawarkan ciri-ciri penyerapan optik
yang lebih baik dan permukaan morfologi filem yang lebih seragam dengan
menghapuskan liang dan gumpalan besar. Peranan lapisan penampan ZnO dalam sistem
campuran tersebut boleh dilihat sebagai suatu ejen yang memudahkan kutipan elektron
dari lapisan aktif ke katod. Dapatan kajian ini menunjukkan bahawa prestasi bagi
peranti berasaskan P3HT:sol-jel ZnO yang disediakan pada komposisi campuran (0.1
ml sol) dan suhu pemanasan optimum pada 100°C dan mengandungi satu lapisan
penampan ZnO, telah meningkat sebanyak 5 kali ganda berbanding dengan peranti
berasaskan P3HT:nanopartikel ZnO.
v
ACKNOWLEDGEMENTS
First and foremost I would like to thank God for the strength He has given to me
that keeps me moving on to accomplish my goal. I would like to thank my family who
have continuously inspired, encouraged and supported me in every trial that come to my
way throughout my life. Also, I thank them for giving me not just financial, but moral
and spiritual support.
I would also like to gratefully acknowledge my supervisor, Dr. Khaulah
Sulaiman for her enthusiastic supervision and valuable guidance to me during this
research work. This work would not have been possible without her support and
assistance. Special thank to my senior, Mr. Fahmi Faraq Muhamad for his precious
encouragement, support, and assistance to me. Besides, I am greatly grateful to all the
officials, staff members and my labmates from Low Dimensional Materials Research
Center, Department of Physics in University of Malaya, for their sincere assistance and
support to me in completing this research work. Special thanks also to all my group
members especially Mr. Lim Lih Wei, Mrs. Zurianti, Mr. Shahino Mah Abdullah, Mr.
Muhamad Saipul and Miss Fadilah for their willingness to share their literature
knowledge and invaluable advice to me.
I would also wish to express my gratitude and thanks to University of Malaya
for providing the sufficient financial support to me in the form of scholarship and
research grants of PS 303/2009C and PS 462/2010B to support my work and to
participate in national and international conferences in different places.
Last but no least, I would like to thank all my friends and housemates for their
understanding, care and encouragement, particularly Miss Nor Khairiah Za’aba, Miss
Siti Hajar, Miss Maisara Othman, Miss Nur Maisarah, Mrs. Noor Hamizah Khanis, and
Mr. Paul Lee Chun Hoong who have continuously given me moral and spiritual support
when it is most required.
vi
RESEARCH PAPERS AND CONFERENCES
A. Published Full Papers (ISI-cited)
1. Yusli, M. N., Toong, W. Y., & Sulaiman, K. (2009). Solvent Effect on the thin
film formation of polymeric solar cells. Material Letters, 63(30), 2691-2694.
2. Toong, W. Y., & Sulaiman, K. (2011). Fabrication and morphological
characterization of hybrid polymeric solar cells based on P3HT and inorganic
nanocrystal blends. Sains Malaysiana, 40(1), 43-47.
B. Conference Papers (Non-ISI cited)
1. Sulaiman, K., & Toong, W. Y., “Studies of optical and morphological properties
on the ZnPc/Gaq3 mixture thin films”. Paper presented at 24th
Regional
Conference on Solid State Science & Technology (RCSSST 2008) in Port
Dickson, Malaysia.
2. Toong, W. Y., & Sulaiman, K., “Fabrication and morphological characterization
of hybrid polymeric solar cells based on P3HT and inorganic nanocrystal
blends”. Paper presented at National Physics Conference (PERFIK 2009) in
Malacca, Malaysia.
3. Toong, W. Y., & Sulaiman, K., “The fabrication and properties characterization
of hybrid polymeric solar cells based on conjugated polymer/inorganic
nanoparticles”. Paper presented at 3rd
International Conference on Functional
Materials and Devices (ICFMD 2010) in Kuala Terengganu, Malaysia.
4. Toong, W. Y., & Sulaiman, K., “The fabrication and properties characterization
of hybrid solar cells based on conjugated polymer/inorganic compound”. Paper
presented at 5th
International Conference on Technological Advances of Thin
Films & Surface Coatings (Thin films and COMPO 2010) in Harbin, China.
vii
TABLE OF CONTENTS
TABLE OF CONTENTS………………………………………………………..
LIST OF FIGURES……………………………………………………………..
LIST OF TABLES………………………………………………………………
LIST OF SYMBOLS……………………………………………………………
LIST OF ABBREVIATIONS…………………………………………………...
vii
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CHAPTER 1: INTRODUCTION……………………………………………..
1.1 Background of the Research Studies………………………………………...
1.2 History of Organic Solar Cells (OSCs)……………………………………...
1.3 The Reasons of Investigation on Hybrid Polymer Solar Cells……………...
1.4 Research Objectives…………………………………………………………
1.5 Dissertation Outline…………………………………………………………
1
1
3
6
9
10
CHAPTER 2: THEORETICAL BACKGROUND…………………………..
2.1 Overview…………………………………………………………………….
2.2 Conjugated Polymers………………………………………………………..
2.3 Charge Transport Characteristics of Conjugated Polymers…………………
2.4 Electronic Properties of Conjugated Polymers……………………………...
2.5 Doping……………………………………………………………………….
2.6 Optical Properties of Conjugated Polymers…………………………………
2.7 Donor Material………………………………………………………………
2.7.1 Poly(thiophene)………………………………………………………….
I) Regioregularity………………………………………………………….
II) Solubility………………………………………………………………..
III) Regioregular Poly (3-hexylthiophene) (P3HT)………………………...
2.8 Acceptor Material……………………………………………………………
2.8.1 Inorganic Nanoparticles………………………………………………...
2.8.2 Physical Properties of Inorganic Nanoparticles………………………...
I) Optical Properties……………………………………………………….
II) Structural and Morphological Properties……………………………….
III) Electrical Properties…………………………………………………….
2.8.3. Types of Inorganic Nanoparticles Used in This Research Work……….
I) Zinc Oxide................................................................................................
II) Titanium Dioxide.....................................................................................
III) Yttrium Oxide…………………………………………………………..
12
12
12
16
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20
22
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2.9 Sol-Gel Synthesis Route…………………………………………………….
2.10 Hybrid Polymer Solar Cells………………………………………….
2.10.1 Basic Working Principles of Hybrid Solar Cells…………………..
2.10.2 Key Parameters of the Hybrid Solar Cells…………………………….
2.10.3 Types of Device Architectures for Organic Solar Cells……………….
I) Single Layer Organic Solar Cell………………………………………...
II) Bilayer Solar Cell……………………………………………………….
III) Bulk Heteronjunction (BHJ) Solar Cell………………………………...
CHAPTER 3: EXPERIMENTAL METHODOLOGY……………………...
3.1 Overview…………………………………………………………………….
3.2 Chemicals and Materials…………………………………………………….
3.2.1. Chemicals and Solutions Preparation…………………………………..
I) P3HT:As-Purchased Inorganic Nanoparticles Blends…………………..
II) P3HT:As-Synthesized Sol-gel ZnO Blend Solutions…………………..
3.2.2 Substrates and Electrodes Preparation………………………………….
3.2.3 Substrates Patterning and Cleaning……………………………………..
3.3 Thin Films Preparation via Spin Coating Technique………………………..
3.3.1 P3HT:ZnO Sol-Gel Film Preparation via Thermal Annealing
Treatment……………………………………………………………...
3.4 Devices Fabrication………………………………………………………….
3.4.1 Single Layer and Bulk Heterojunction (BHJ) Structures……………….
3.4.2 Aluminum (Al) Electrodes Deposition via Thermal Evaporation……...
3.5 Characterization Techniques………………………………………………...
3.5.1 Ultraviolet-Visible-Near Infrared (UV-VIS-NIR) Spectrophotometer…
3.5.2 X-ray Diffraction (XRD) Technique……………………………………
3.5.3 Atomic Force Microscopy (AFM)……………………………………...
3.5.4 Field-Emission Scanning Electron Microscopy (FESEM)……………..
3.5.5 Surface Profilometer………………………………………………...….
3.5.6 Photovoltaic (PV) Measurement………………………………………..
CHAPTER 4: CHARACTERIZATION OF P3HT:ZNO HYBRID THIN
FILMS AND SOLAR CELL DEVICES……………………..
4.1 Overview…………………………………………………………………….
4.2 Optical Characterization: Ultraviolet-Visible-Near Infrared (UV-Vis-NIR)
Spectra Analysis…………………………………………………………….
4.3 Structural Characterization: X-ray Diffraction (XRD) Spectra Analysis…...
34
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ix
4.4 Morphological Characterization: Atomic Force Microscopy (AFM) and
Field Emission Scanning Electron Microscopy (FESEM) Analysis………..
4.4.1 AFM Characterization…………………………………………………..
4.4.2 FESEM Imaging………………………………………………………...
4.5 Electrical Characterization: Current Density-Voltage (J-V) Curve Analysis.
CHAPTER 5: HYBRID SOLAR CELLS BASED ON INORGANIC
NANOPARTICLES AND P3HT:ZNO ACTIVE
LAYERS PREPARED BY SOL-GEL SYNTHESIS
ROUTE………………………………………………………
5.1 Overview…………………………………………………………………….
5.2 Part I: Investigation of Hybrid Solar Cells Based on Various Inorganic
Nanoparticles………………………………………………………………..
5.2.1. Results on P3HT:TiO2 Blend Films and Their Based Solar Cell
Devices…………………………………………………………………
A. Optical Characterization………………………………………………...
B. Structural Characterization……………………………………………...
C. Morphological Characterization………………………………………...
D. Electrical Characterization……………………………………………...
5.2.2. Results on P3HT:Y2O3 Blend Films and Solar Cell Devices………….
A. Optical Characterization………………………………………………...
B. Structural Characterization……………………………………………...
C. Morphological Characterization………………………………………...
D. Electrical Characterization……………………………………………...
5.2.3 Comparison of Hybrid Systems Based on Three Different Types of
Inorganic Metal Oxide Nanoparticles…………………………………..
A. Optical Characterization………………………………………………..
B. Structural Characterization……………………………………………...
C. Morphological Characterization………………………………………..
D. Electrical Characterization……………………………………………...
5.3 Part II: The Improvement of P3HT:ZnO Devices by Sol-gel Synthesis
Route………………………………………………………………………...
5.3.1 First Approach: Effects of Different Sol Content………………………
A. Optical Characterization………………………………………………...
B. Structural Characterization……………………………………………...
C. Morphological Characterization………………………………………...
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x
D. Electrical Characterization……………………………………………...
5.3.2 Second Approach: Effects of Different Annealing Temperature……….
A. Optical Characterization………………………………………………...
B. Morphological Characterization………………………………………...
C. Electrical Characterization……………………………………………..
5.3.3 Third Approach: Effects of Additional ZnO Buffer Layer……………..
A. Morphological Characterization………………………………………...
B. Electrical Characterization……………………………………………...
CHAPTER 6: CONCLUSIONS AND FUTURE WORKS………………….
6.1 Conclusions………………………………………………………………….
6.2 Future Works………………………………………………………………...
149
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xi
LIST OF FIGURES
Page
Figure 1.01: Block diagram of research methodology used in this study…....... 11
Figure 2.01: The examples of 1) non-conjugated polymers: (i) Polypropylene,
(ii) Poly(vinyl alcohol); 2) conjugated polymers: (i)
Polyacetylene, (ii) Polythiophene (Skotheim, et al.,
1998)……………………………………………………………...
14
Figure 2.02: (a) The structure of ethylene comprises of σ bonds which formed
from the three sp2-hybridized orbitals on each carbon atom. (b)
The formation of π bond due to the overlap of the unhybridized
pz orbital. (c) A cutaway view of the whole σ and π system
within the ethylene molecule (Hari Singh Nalwa,
2002)……………………………………………………………...
15
Figure 2.03: The schematic diagram of HOMO and LUMO bands…………...
15
Figure 2.04: Schematic representation of intrachain charge diffusion (left) and
interchain charge diffusion (right) in polyacetylene……………...
16
Figure 2.05: Potential energy as a function of bond length alternation for the
two categories of conjugated polymers are exhibited for (a) a
degenerate ground state conjugated polymer, trans-
polyacetylene; (b) a non-degenerate ground state conjugated
polymer (in the given example is poly-para-phenylene (PPP))…..
18
Figure 2.06: Schematic structure and energy diagram for solitons in
conjugated polymer is shown…………………………………….
19
Figure 2.07: Schematic structure and band diagrams for excitations in non-
degenerate ground state conjugated polymers. Allowed optical
transitions are exhibited by the blue-colored dashed arrows…......
20
Figure 2.08: Removal of two electrons (p-type doping) from a polyacetylene
chain produces two radical cations. The combination of both
radicals forms a spinless di-cation………………………………..
21
Figure 2.09: Removal of two electrons (p-type doping) from a polythiophene
chain produces bipolaron. Bipolaron moves as a unit up and
down the polymer chain, which is responsible to the electrical
properties of conjugated polymers……………………………….
22
Figure 2.10: A coplanar π-orbitals polythiophene (top); a twisted substituted
polythiophene (bottom) (Skotheim, et al., 1998)………………...
23
Figure 2.11: The chemical structure of monomer repeating unit of PTs……....
24
Figure 2.12:
The differences between regiorandom and regioregular PTs in
aspect of chain structure and charge transport characteristics…....
25
xii
Figure 2.13: The chemical structure of P3HT………………………………….
27
Figure 2.14: The structures and shapes of the inorganic nanoparticles that are
widely used in hybrid polymer solar cells………………………..
29
Figure 2.15: Energy dispersion for the (a) bulk semiconductor compared with
that of (b) the nanoparticles (Dhlamini, et al., 2008)……………..
31
Figure 2.16: The chemical structure of ZnO……………………………….......
32
Figure 2.17: The chemical structure of TiO2……………………………….......
33
Figure 2.18: The chemical structure of Y2O3…………………………………..
34
Figure 2.19: Schematic structure of hybrid solar cell………………………….
36
Figure 2.20: The working principle of hybrid polymer solar cell consisting of
an electron donor and acceptor pair………………………...........
38
Figure 2.21: The current density-voltage curve (J-V curve) of a hybrid solar
cell under illuminated condition (dash line)
……………………………………………………………………
39
Figure 2.22: (a) The basic structure of a single layer solar cell. (b) Schematic
of a single layer solar cell with a Schottky contact at the lower
function electrode B contact. Photogenerated excitons can only
be dissociated within a narrow deplection layer, and thus the
device is exciton diffusion limited……….....................................
41
Figure 2.23: The basic structure of a bilayer solar cell. (b) Schematic of a
bilayer heterojunction solar cell. The donor (D) contacts the
higher work function electrode A and the acceptor (A) contacts
the lower work function electrode B, in order to achieve charge
carrier collection, respectively. Excitons can only be dissociated
within the region at the D/A interface…………………………....
42
Figure 2.24: (a) The basic structure of a bulk heterojunction solar cell. (b)
Schematic of a bulk heterojunction solar cell device. The
excitons can be dissociated throughout the volume of material as
the D is well blended with A……………………………………..
43
Figure 3.01: Pristine solutions and the P3HT:inorganic nanoparticles blend
solutions…………………………………………………………..
46
Figure 3.02: P3HT:sol-gel ZnO blend solutions.................................................
47
Figure 3.03: ITO substrates patterning………………………………………....
49
Figure 3.04: The spin coating machine which is used for thin films
deposition…………………………………………………………
51
xiii
Figure 3.05: Schematic representation of the spin coating technique………….
51
Figure 3.06: The flow chart for the preparation of the P3HT:sol-gel ZnO
films……………………………………………………………....
52
Figure 3.07: The device construction of solar cells based on different devices
geometry and hybrid systems…………………………………….
54
Figure 3.08: Thermal evaporation system for Al electrode deposition. The
inset shows the designed shadow mask used in this work……….
55
Figure 3.09: The schematic diagram of the evaporation system arrangement
within the vacuum chamber………………………………………
55
Figure 3.10: Possible electronic transitions of π, σ or n electrons……………..
58
Figure 3.11: Photograph of Jasco V-570 UV-VIS-NIR Spectrophotometer…..
59
Figure 3.12: Schematic diagram of the components of a UV-VIS-NIR
spectrophotometer………………………………………………..
60
Figure 3.13: The diffraction pattern of X-rays by planes of atoms…………….
61
Figure 3.14: (a) Photograph of the XRD instrument (Bruker AXS). (b) The
basic components of a X-ray diffractometer……………………..
62
Figure 3.15: Schematic diagram of an atomic force microscope………………
64
Figure 3.16: Photograph of Veeco Dimension 3000 AFM instrument………...
66
Figure 3.17: Field-emission scanning electron microscope (FESEM)………...
67
Figure 3.18: Principle features of a SEM instrument…………………………..
68
Figure 3.19: (a) The schematic diagram of a contact profilometer. (b)
Resultant surface profile that is generated based on the tip
deflection…………………………………………………………
70
Figure 3.20: KLA Tensor P-6 surface profilometer for film thickness
measurement……………………………………………………...
70
Figure 3.21: (a) An Oriel 67005 solar simulator. (b) The internal structure of
the Oriel solar simulator………………………………………….
72
Figure 3.22: (a) A Keithley 236 SMU instrument. (b) A solar cell device
connected to its appropriate terminals under I-V measurement….
73
Figure 4.01: Absorption coefficient of pristine P3HT, ZnO and P3HT:ZnO
blend films with different contents of ZnO nanoparticles. The
inset shows the variation of maximum absorption coefficient
peak values, αmax and film thickness, t as a function of blend
composition……………………………………………………….
74
xiv
Figure 4.02: The pre-estimated Eg of the pristine (a) P3HT film and (b) ZnO
nanoparticles film using plots of dln(αhν)/dhν versus hν………...
79
Figure 4.03: Plots of ln(αhν) versus ln(hν − Eg) to determine the n value for
(a) P3HT and (b) ZnO nanoparticles films, respectively…………
80
Figure 4.04: Plots of (αhν)2 against hν for (a) P3HT and (b) ZnO films………
81
Figure 4.05: The typical energy band diagram of (a) P3HT and (b) ZnO
nanoparticles films where the conduction band of ZnO is tunable
due to the variation in nanoparticles size…………………………
82
Figure 4.06: Plots of dln(αhν)/dhν versus hν to pre-estimate the Eg of the
P3HT:ZnO blend films with different contents of ZnO………….
83
Figure 4.07: Plots of of (αhv)2 against hν for P3HT:ZnO blend films with
different contents of ZnO………………………………………....
84
Figure 4.08: The XRD pattern of ZnO nanoparticles in powder form………....
85
Figure 4.09: Williamson-Hall plots to determine the microstrain of ZnO……..
87
Figure 4.10: The XRD spectra of pristine P3HT and P3HT:ZnO blend films.
The inset illustrates the orientation of P3HT crystallites with
respect to the substrate……………………………………………
87
Figure 4.11: Three-dimensional AFM images by 10 × 10 μm2 scan for (a)
pristine P3HT and the blend films in different compositions with
(b) 1%, (c) 2%, (d) 3%, (e) 4%, (f) 5%, and (g) 10% ZnO………
93
Figure 4.12: 2-dimensional AFM images of the (a) pristine P3HT and its
blends with (b) 1%, (c) 2%, (d) 3%, (e) 4%, (f) 5%, and (g) 10%
ZnO……………………………………………………………….
97
Figure 4.13: The schematic illustration of some possible morphologies that
most likely have been achieved for the P3HT:ZnO blends
incorporated with different amount of ZnO contents…………….
99
Figure 4.14: FESEM images of (a) P3HT and its blends with (b) 1%, (c) 3%,
(d) 5% and (e) 10% ZnO ((e(ii) shows the focused zone area of
image (e))………………………………………………………....
101
Figure 4.15: The FESEM images of pristine ZnO nanoparticles film…………
101
Figure 4.16: The current density–voltage (J-V) characteristics of solar cell
devices based on pristine P3HT and P3HT:ZnO blends in
different blend compositions under dark condition………………
102
Figure 4.17: Current density-voltage (J-V) characteristic curves of the solar
cell devices under light illumination. The inset displays an
equivalent circuit diagram of solar cells which consists of series
xv
resistance, Rs and shunt resistance, Rsh…………………………..
103
Figure 4.18: The schematic energy band diagram of
ITO/PEDOT:PSS/P3HT:ZnO/Al devices………………………...
105
Figure 4.19:
The variation of (i) Jsc and Voc, (ii) FF and η with the blend
composition……………………………………………………….
106
Figure 4.20: The variation of Rsh and Rs with the blend composition………....
109
Figure 5.01: Absorption coefficient of P3HT:TiO2 blend films with different
contents of TiO2 nanoparticles. The inset shows the variation of
maxmimum absorption coefficient peak values, αmax and film
thickness, t as a function of TiO2 contents…………………….....
112
Figure 5.02: The XRD patterns of (i) TiO2 in powder form; (ii) pristine P3HT
and P3HT:TiO2 blend films……………………………...............
113
Figure 5.03: AFM images in 2D and 3D views for (a) pristine P3HT and
P3HT:TiO2 blend films with (b) 1%, (c) 2%, (d) 3%, (e) 4%, (f)
5%, and (g) 10% TiO2....................................................................
116
Figure 5.04: The J-V plots for P3HT:TiO2 solar cells with different blend
compositions under light illumination……………………………
117
Figure 5.05: Variations in the device parameters for (i) Jsc and Voc, (ii) Rs and
Rsh, (iii) FF and η as a function of blend composition…………...
118
Figure 5.06: The optical spectra for the P3HT:Y2O3 blend films. The inset
indicates the variation of αmax and film thickness as a function of
Y2O3 content in P3HT:Y2O3 blends………………………………
119
Figure 5.07: The XRD spectra of (i) Y2O3 nanopowder; (ii) P3HT:Y2O3 blend
films………………………………………………………………
120
Figure 5.08: The AFM images for (a) pristine P3HT and the blend films with
(b) 1%, (c) 2%, (d) 3%, (e) 4%, (f) 5%, and (g) 10% Y2O3...........
123
Figure 5.09: The J-V plots for pristine P3HT and P3HT:Y2O3 blend devices....
124
Figure 5.10: Variations in the device parameters for (i) Jsc and Voc, (ii) Rs and
Rsh, (iii) FF and η as a function of blend composition…………...
125
Figure 5.11: (i) Plots of (αhν)2 against hν for ZnO, TiO2 and Y2O3 films, the
inset shows the plots of ln(αhν) versus ln(hν − Eg) to determine
the type of electronic transition for the nanoparticles. (ii)
Absorption coefficient spectra of the P3HT:nanoparticles blends;
the inset shows the estimated Eg of the blends………...................
126
Figure 5.12: The XRD spectra of P3HT:nanoparticles blend films……………
128
xvi
Figure 5.13: The AFM images of P3HT:nanoparticles blend films in 3D and
2D views for (i) P3HT:ZnO, (ii) P3HT:TiO2, and (iii)
P3HT:Y2O3 film…………………………………………………..
129
Figure 5.14: FESEM images of the (a) pristine inorganic metal oxide
nanoparticles, and (b) P3HT:nanoparticles blend films with ZnO
(I), TiO2 (II) and Y2O3 (III)……………………………………….
131
Figure 5.15: Comparison of the J-V characteristics of the solar cells based on
three different types of P3HT:metal oxide nanoparticles hybrid
systems under white light illumination…………………………...
133
Figure 5.16: An inferred schematic energy band diagram for the
P3HT:inorganic nanoparticles hybrid systems, placed between
anode ITO and cathode Al………………………………………..
135
Figure 5.17: Absorption coefficient spectra of pristine P3HT, sol-gel derived
ZnO and P3HT:sol gel ZnO blend films with different sol
contents. The inset indicates the variation of αmax and film
thickness as a function of sol content. Also, the comparison
between P3HT:ZnO NPs and P3HT:sol-gel ZnO films is
shown……………………………………………………………..
137
Figure 5.18: The plots of (αhν)2 against hν for P3HT:sol gel ZnO blend films
with different sol contents………………………………..............
138
Figure 5.19: The XRD patterns of P3HT:sol-gel ZnO blend films with
different sol contents as well as XRD patterns of bared ZnO NPs
deposited film and sol-gel derived ZnO films…………………....
140
Figure 5.20: XRD parameters of blend films as a function of different sol
contents, namely: (i) FWHM, (ii) crystallite size and (iii)
dislocation density corresponding to (100), (002), and (110)
planes……………………………………………………………..
141
Figure 5.21: The AFM images of P3HT:sol-gel ZnO blend films with (a) 0.05
ml, (b) 0.1 ml, (c) 0.2 ml, and (d) 0.3 ml sol content…….………
144
Figure 5.22: AFM images by 10 × 10 µm2 scan in both 2D and 3D views of
(i) P3HT:ZnO NPs and (ii) P3HT:sol gel ZnO blend films………
144
Figure 5.23: The FESEM images of P3HT:sol-gel ZnO films with (a) 0.05 ml,
(b) 0.1 ml, (c) 0.2 ml, and (d) 0.3 ml sol…………………………
144
Figure 5.24: The comparison of FESEM images between (i) P3HT:ZnO NPs
film and (ii) P3HT:sol-gel ZnO film. The inset shows the surface
micrograph of (i) bare ZnO NPs film and (ii) bare sol-gel derived
ZnO film………………………………………………………….
147
Figure 5.25: The J-V plots for P3HT:sol-gel ZnO solar cells with different sol
contents in blends………………………………………………...
147
xvii
Figure 5.26:
Variations in the device parameters for (i) Jsc and Voc, (ii) Rs and
Rsh, (iii) FF and η as a function of sol content in blends………....
149
Figure 5.27: Photovoltaic comparison for P3HT:sol gel ZnO and P3HT:ZnO
NPs films………………………………………………………....
151
Figure 5.28: The optical spectra of P3HT:sol-gel ZnO films annealed at
different temperatures, Ta. The inset indicates the variation of
αmax and film thickness as a function of Ta……………………….
153
Figure 5.29: The plots of (αhν)2 against hν for P3HT:sol gel ZnO blend films
annealed at different Ta…………………………………………...
153
Figure 5.30: The AFM images in 2D and 3D of P3HT:sol gel ZnO films
annealed at different Ta of (a) 75 °С, (b) 100 °С, (c) 150 °С, (d)
175 °С…………………………………………………….............
155
Figure 5.31: The J-V plots for P3HT:sol gel ZnO solar cells fabricated from
active layers annealed at different Ta……………………………..
156
Figure 5.32: Variation in the device parameters for (i) Jsc and Voc, (ii) Rs and
Rsh, (iii) FF and η as a function of annealing temperature………..
157
Figure 5.33: Morphological characteristics of the P3HT:sol-gel ZnO film (i)
without and (ii) with an additional ZnO buffer layer (BL) via
AFM imaging…………………………………………………......
159
Figure 5.34: The comparison of the J-V characteristics of the solar cell
devices without and with an additional ZnO buffer layer……......
160
xviii
LIST OF TABLES
Page
Table 1.1: A list of development in the organic solar cells………………. 5
Table 3.1: Blend compositions and the respectively masses of
P3HT:inorganic nanoparticles blends………………………….
45
Table 3.2: Blend compositions of P3HT:sol-gel ZnO…………………….
47
Table 3.3: The deposition parameters of Al electrodes…………………...
56
Table 3.4: Parameters of UV-VIS-NIR spectrum…………………………
60
Table 3.5: Scanning parameters of XRD measurement…………………...
63
Table 3.6: Parameters of the AFM measurement…………………………
66
Table 3.7: Scanning parameters of film thickness measurement………….
71
Table 4.1: The comparison of the absorption coefficient peak values and
the corresponding wavelength positions for pristine P3HT and
P3HT:ZnO blends……………………………………………...
76
Table 4.2: The values of average thickness for pristine P3HT, ZnO and
P3HT:ZnO blend films…………………………………………
76
Table 4.3: The values of Eg for pristine P3HT, ZnO and P3HT:ZnO blend
films based on two different types of functions, in which the
later one gives a more precise estimation value of Eg………….
84
Table 4.4: Summary of the XRD properties of the P3HT:ZnO
nanoparticles blend films with different ZnO content………....
89
Table 4.5: The mean surface roughness and root-mean-square roughness
of the films obtained from AFM for Figure 4.11……………....
94
Table 4.6: The comparison of device characteristics parameters for
pristine P3HT and P3HT:ZnO solar cells with different ZnO
concentration…………………………………………………...
104
Table 4.7: The variation of Rsh and Rs with the blend composition………..
109
Table 5.1: Summaries of the αmax values together with the corresponding
wavelength positions, λ, film thickness, t and estimated Eg for
pristine P3HT and P3HT:TiO2 blends………………………….
112
Table 5.2: Summaries of the Bragg diffraction angles and film
roughnesses (obtained from AFM images) of pristine P3HT
and P3HT:TiO2 blends…………………………………………
113
Table 5.3: The comparison of device characteristics parameters for
P3HT:TiO2 solar cells………………………………………….
117
xix
Table 5.4: Summaries of the αmax values together with the corresponding
wavelength positions, λ, film thickness, t and estimated Eg for
pristine P3HT and P3HT:Y2O3 blends…………………………
119
Table 5.5: Summaries of the Bragg diffraction angles and film
roughnesses of pristine P3HT and P3HT:Y2O3 blends………...
121
Table 5.6: The comparison of device characteristics parameters for P3HT
and P3HT:Y2O3 solar cells……………………………………..
124
Table 5.7: Variation in the J-V characteristics of P3HT: metal oxide
nanoparticles hybrid solar cells………………………………...
133
Table 5.8: The comparison of αmax values, the corresponding wavelength
positions, average film thicknesses and estimated Eg for
P3HT:sol gel ZnO blends with different sol contents………….
139
Table 5.9
(i) & (ii):
Summaries of the XRD parameters of P3HT:sol gel ZnO blend
films with different sol contents……………………………….
140
Table 5.10: Surface roughness values of the blend films obtained from
AFM……………………………………………………………
145
Table 5.11: The comparison of device characteristics parameters for
P3HT:sol-gel ZnO solar cells…………………………………..
149
Table 5.12: The comparison of the αmax values, the corresponding
wavelength positions, average film thicknesses and estimated
Eg for the P3HT:sol gel ZnO films annealed at different Ta…...
153
Table 5.13: Surface roughness values of P3HT:sol gel ZnO films annealed
at different Ta…………………………………………………..
156
Table 5.14: The comparison of device characteristics parameters for
P3HT:sol gel ZnO solar cells…………………………………..
156
Table 5.15: Surface roughness values of the films obtained from AFM……
160
Table 5.16: Device characteristics parameters of P3HT:sol gel ZnO solar
cells with ZnO buffer layer spun at different spin speed………
160
Table 5.17: Device characteristics parameters of P3HT:sol gel ZnO solar
cells……………………………………………………………..
161
xx
LIST OF SYMBOLS
> Less than
< More than
π Pi
σ Sigma
T Transmittance
A Absorbance
Io Light intensities
E Energy
λ Wavelength
d Interatomic spacing distance
θ Diffraction angle
Eg Energy gap
Δr Bond length alternation
ISC Short-circuit current
VOC Open circuit voltage
Pmax Maximum power
Pout Output power
Vmax Voltage at maximum power
Imax Current at maximum power
Pin Input power
FF Fill factor
η Power conversion efficiency
α Absorption coefficient
t Film thickness
B Full width at half maximum
tc Crystallite size
C Scherrer constant
ε Strain
ΛE Exciton diffusion length
DE Exciton diffusion coefficient
τE Exciton lifetime
Ra Mean surface roughness
Von Turn-on voltage
Jsc Short-circuit current density
Rsh Shunt resistance
Rs Series resistance
D/A Donor/Acceptor
J-V Current density-voltage
δ Dislocation density
Ta Annealing temperature
xxi
LISTS OF ABBREVIATIONS
BHJ Bulk heterojunction
1-D One dimensional
AFM Atomic force microscopy
Ag Silver
Al Aluminum
AM 1.5 Air mass 1.5
Au Gold
BL Buffer layer
C60 Buckminsterfullerene
Ca Calcium
CB Conduction band
CdSe Cadmium selenide
CHCl3 Chloroform
Cu Copper
DEA Diethanolamine
EDX X-ray spectroscopy
FESEM Field-effect scanning electron microscopy
FWHM Full width at half maximum
HCl Hydrochloric acid
HE High energy
HOMO Highest occupied molecular orbital
ITO Indium tin oxide
JCPDS Joint committee on powder diffraction standards
LE Low energy
LUMO Lowest unoccupied molecular orbital
MEH-PPV Poly[2-methoxy-5-(2’-ethylhexyloxy)-p-phenylene vinylene]
MgPh Magnesium phthalocyanine
NPs Nanoparticles
OSCs Organic solar cells
P3HT Poly(3-hexylthiophene)
PbS Plumbum sulfide
PCBM. [6,6]-phenyl-C61-butyric acid methyl ester
PCE Power conversion efficiency
PEDOT:PSS Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)
PPP Poly-para-phenylene
PT Poly(thiophene)
PV Photovoltaic
RMS Root-mean-square
rpm Rotations per minute
SMU Source measuring unit
STC Standard test condition
TiO2 Titanium dioxide
UV-VIS-NIR Ultraviolet-visible-near-infrared
VB valence band
XRD X-ray diffraction
ZnO Zinc oxide
xxii
